Current mobile satellite communication systems, such as Iridium, Globalstar, and ICO, utilize low-cost user terminals as one of their key system features. To maintain communications linkage with these current mobile systems, the system satellites provide multiple beam and high-gain services to the subscribers. The low-cost and low-gain hand-held terminals utilized by the users of these systems, transmit and receive signals to and from high performance satellites which populate almost the entire hemisphere. Some of these current systems require access to at least two satellites to assure a soft hand-over process as the satellites progress from horizon to horizon. As a result, the satellite system becomes more reliable and available as more satellites come into a user's field of view (FOV). The satellite constellations provided by these current systems are thus sized to guarantee a minimum number of satellites within a user's FOV over large coverage areas at all times.
All of these current mobile satellite communication systems, however, suffer from certain disadvantages. First, they all have limited frequency (the term “frequency” is generalized here to refer to frequency, time slot or CDMA code) resources. Any given frequency over a given ground position can only be utilized by one user at a time. Thus, if one user accesses a satellite using a particular frequency to communicate to his counterpart on the network, other satellites and/or users in the same region cannot reuse the same frequency resource in the same local area. In particular, if a nearby secondary user has a handset that requires the same frequency resource as is being utilized by the first user, the second user is unable to access the system, even via different satellites. This is true regardless of the sophistication of the system, including systems that utilize multiple beam satellite designs. Even when multiple satellites are available at a given geographic location, the same frequency spectrum cannot be used by more than one limitations on frequency re-use for point-to-point communications.
It is another object of the present invention to provide a wireless communication system that utilizes individual transponders and mobile terminals that are relatively simple and of low complexity.
It is a further object of the present invention to provide a wireless communication system with high system reliability through graceful degradation.
It is still another object of the present invention to provide a wireless communication system wherein the complexity of the system is concentrated at the central processing hub stations.
It is yet another object of the present invention to provide a wireless communication system with more accurate capabilities for satellite and/or user positioning.
In accordance with the objects of the present invention, a mobile wireless communication system is provided. The mobile wireless communications system includes a plurality of individual communication transponding platforms. The plurality of individual transponders are each in user in a local area. The availability of multiple satellites merely serves to increase the availability of the system to the user. However, the total capacity of these mobile communication satellite systems is still limited by their inefficient usage of the available frequency resources. Thus, the potential growth of these current satellite communication systems is inherently limited.
Additionally, current telecommunications systems generally allow only mobile-to-hub and hub-to-mobile communications in most low earth orbit and medium earth orbit mobile satellite constellations. Mobile-to-mobile linkages require multiple hops between hubs. This means that two or more frequency resources must be committed by the system to close the link.
It is clearly desirable to provide a mobile communication satellite system that relaxes the above constraints, and more efficiently utilizes current mobile satellite communication system resources, while also providing much greater opportunity for system growth.